This invention relates to a method of drilling a hole into a workpiece with a laser, and to apparatus for carrying out the method.
The technology of laser-drilling holes is established, but one particular problem arises when a hole is to be drilled through a workpiece into a cavity in the workpiece. This problem arises because the drilling of the hole then enables the laser beam passing through the hole to strike the workpiece on the opposite side of the cavity, causing damage to the workpiece and conceivably drilling a further hole through it. An attempt to deal with this problem has been to insert a metal pin in the cavity to obstruct the laser beam, but the pin is then damaged by the laser beam and has to be replaced, debris from the pin has to be cleared, and the design of the cavity can make the insertion of a pin difficult.
A particular workpiece to which this invention may be applied is a fuel injection nozzle, especially for diesel fuel, but there are many other industrial applications, of which some are exemplified later in this specification.
It is an object of the present invention to provide an improved method of laser-drilling a hole through a workpiece into a cavity in the workpiece, and to provide apparatus for carrying out the method.
In accordance with the present invention, a method of laser-drilling a hole through a workpiece into a cavity in the workpiece comprises drilling a hole through a workpiece with a laser, providing a fluid having laser-barrier properties in the cavity so that, when the hole is open to the cavity, laser light passing through the hole is incident upon the fluid whereby the workpiece across the cavity from the hole is protected from the laserlight, and arranging that the fluid does not enter the laser-drilled hole during the drilling process.
Preferably a method of laser-drilling a hole through a workpiece into a cavity in the workpiece comprises substantially filling the cavity in alignment with the hole to be drilled with a fluid having laser-barrier properties, drilling the hole with a laser, and arranging that the fluid does not enter the hole when the hole is open to the cavity.
The fluid may have laser-light scattering properties, in which case the fluid may be a liquid in which case the liquid may be a colloid, or the fluid may be gas-borne particles such as a smoke as an example, or liquid droplets. Generally the term xe2x80x9cfluidxe2x80x9d covers anything that can flow, including a fluidised particulate bed, eg. of carbon particles, or ceramic particles or metal particles.
The fluid may have laser-light absorbing properties in which case the fluid preferably absorbs laser-light as thermal energy and may be a particulate suspension such as a liquid suspension of pigment particles as an example.
If the fluid has laser-light absorbing properties, it may fluoresce to re-emit the energy of incident laser light at a different frequency, and is preferably a liquid solution of a dye-stuff.
Preferably the pressure of the fluid in the cavity is arranged to be sufficiently low that the fluid does not enter the hole when the hole is open to the cavity.
Preferably also the fluid is a liquid and its surface tension and/or contact angle adjacent to the hole is taken into account in arranging the pressure of the liquid to be sufficiently low that the liquid does not enter the hole when the hole is open to the cavity.
Preferably the pressure of the fluid in the cavity is less than the ambient pressure external of the workpiece.
The method may include directing a gas stream at the workpiece at the same place as the laser beam and arranging that the pressure of the fluid in the cavity is less than the combination of the ambient pressure external of the workpiece and the pressure applied to the workpiece by the gas stream.
Preferably at least some of the fluid in the cavity is passed across alignment with the hole to be drilled.
Preferably also the fluid is recirculated in the cavity.
Preferably further the cavity is provided with a fluid flow pipe, and fluid is supplied to or extracted from the cavity through the fluid flow pipe to aid recirculation of the fluid in the cavity.
Preferably further fluid is pumped into the cavity and fluid is pumped out of the cavity.
Preferably further the fluid is supplied to the cavity through the fluid flow pipe, and the fluid flow pipe is located in such a position that it is out of alignment with the hole to be drilled in the workpiece, but the stream of fluid issuing from the fluid flow pipe comes into such alignment before its flow loses its integrity.
Preferably further the interior shape of the hole is drilled by orbitting the laser beam repeatedly around the desired path.
The method may further comprise providing a separator between the fluid and the hole to control the interaction of the fluid and scattered laser light with the hole. For example, when the fluid is in contact with the hole and if the fluid is heated by absorption of the laser beam, then the heat of the fluid can be transferred to the hole or the area around the hole. This can cause melting of the workpiece surface in the hole and its vicinity. When the fluid with laser-barrier properties has laser light scattering properties, then the scattered light can have sufficient intensity to melt or vaporise the workpiece. Therefore, if the fluid were to enter the hole, the scattering can enlarge the diameter and/or change the shape of the hole. It can also cause the edge of the hole in the cavity to become shaped or rounded. There are many other ways in which the interaction of the laser and fluid can affect the hole and the workpiece and these effects can be advantageous or disadvantageous. Therefore, this invention gives a method and apparatus for controlling these effects whilst protecting the cavity wall from damage by the laser beam.
Alternatively the method further comprises at least partially inserting a separator between the fluid and the hole so that when the hole is open to the cavity, the interaction of the fluid and the laser light with the hole is controlled.
The fluid may have cooling properties in addition to laser-light absorbing and/or laser-light scattering properties.
A volume of gas may be used as the separator to control the interaction between the fluid and scattered laser-light with the hole.
Preferably a stream of gas is used as the separator to control the interaction between the fluid and scattered laser-light with the hole.
A solid barrier may be used as the separator to control the interaction between the fluid and scattered laser-light with the hole.
Preferably the solid barrier is perforated with a hole or holes or a slot or slots.
Preferably also a hole or slot is in alignment with the laser beam.
Preferably also the solid barrier also prevents the fluid from splashing back or ingressing onto or into the laser drilled hole.
Light scattering properties can also be achieved by using a fluid which has micro-cavitation. The micro-cavitation forms micro-bubbles in the fluid. The micro-bubbles act like micro-lenses which cause the light incident on the micro-bubble to be focused so that it diverges rapidly. The large number of micro-bubbles have an overall effect of scattering the light into a wide range of angles so that the laser light hitting the back wall of the cavity is very diffuse and of low intensity so that it does not machine or damage the back wall. Aids to achieving micro-cavitation can be flowing the fluid in a certain way, adding other fluids or detergents or other media to the main fluid, or applying ultrasound to the fluid delivery system.
The barrier-fluid or cooling fluid may be a single fluid or a combination of fluids. The fluid or fluids may have a combination of laser-light absorbing, laser-light scattering properties and thermal cooling properties.
The complete or partial separator between the fluid and the hole may be another fluid (gas or liquid) or a solid media such as glass or crystal or ceramic or metal or plastic. It may also be some combination of materials/media.
The invention may include directing the barrier-cooling fluid so that its interaction with the hole can be controlled. For example, the fluid may be directed at the hole to increase the interaction or directed away from the hole to reduce or eliminate the interaction. Increasing the interaction may be used to enlarge the hole or shape the hole or change the surface of the hole or the cavity around the hole or some combination of these effects. Eliminating the interaction can be used to ensure sharp edges to the hole (small edge radius) where the hole breaks into the cavity.
The invention may include the use of precision machined components which fit inside the cavity and are used to deliver the barrier fluid in the required manner. These inserts may also form part of the separator or be used to deliver the fluid which forms the separator or some combination of the two. These inserts may be in the form of tubes or channels or guides or baffles and may be made from metal or ceramic or glass or crystal or plastic or any other suitable material. The insert can be constructed so as to allow the laser beam to impinge upon the fluid but have baffles and guides to prevent the fluid from splashing or ingressing onto or into the laser drilled hole.
The inserts may be used in combination with the barrier-fluids to protect the cavity wall from damage. That is, whereas a metal pin has been shown to be unsatisfactory, a metal pin with a barrier-fluid passing over its surface or inside it can be satisfactory since the barrier-fluid protects and cools the metal pin. When the insert is used in this mode, then preferably the insert is a material which is not easily damaged by the laser. The insert may be made from metal, ceramic, glass, crystal, plastic, rubber or diamond or another material.
If a pin or insert is made from a material that is easily damaged by the laser, then it should be easy and cheap to replace. For example, a graphite rod in an oxygen rich atmosphere which vapourizes when struck by the laser beam and reacts to form carbon dioxide or monoxide gas. The graphite rod could be continuously fed into the cavity and the gas extracted thereby giving protection to the cavity from the laser beam.
In accordance with the invention apparatus for laser-drilling a hole through a workpiece into a cavity in the workpiece comprises fluid supply means for providing a fluid having laser-barrier properties in the cavity so that, when the hole is open to the cavity, laser light passing through the hole is incident upon the fluid whereby the workpiece across the cavity from the hole is protected from the laser light, the arrangement being such that the fluid does not enter the laser-drilled hole during the drilling process.
Preferably apparatus for laser-drilling a hole through a workpiece into a cavity in the workpiece comprises a holder for the workpiece, fluid supply means for substantially filling the cavity in alignment with the hole to be drilled with a fluid having laser-barrier properties, and control means for arranging that the fluid does not enter the hole when the hole is open to the cavity.
Preferably said control means comprises pressure or flow regulator means to arrange that the pressure of the fluid in the cavity is sufficiently low that the fluid does not enter the hole when the hole is open to the cavity.
Preferably also said fluid supply means comprises flow means to pass at least some of the fluid in the cavity across alignment with the hole to be drilled.
Preferably further the flow means is able to recirculate fluid in the cavity.
Preferably further the flow means comprises a fluid flow pipe to be inserted in the cavity for fluid to be supplied to or extracted from the cavity through the fluid flow pipe to aid recirculation of the fluid in the cavity.
Preferably further the apparatus includes a first pump for pumping fluid into the cavity, a second pump for pumping fluid out of the cavity, and a fluid reservoir.
The second pump may be a vacuum pump, exhausting the reservoir to atmosphere, or the second pump may be a pressure pump for pumping fluid out of the cavity to the reservoir.
Preferably further the fluid flow pipe is located in such a position that it will be out of alignment with the hole to be drilled in the workpiece, but the stream of fluid that will issue from the fluid flow pipe in use will cross such alignment before its flow loses its integrity.
Preferably further the apparatus also includes a supply of a fluid having laser-barrier properties and a laser.
Preferably also the laser is a gas laser such as a copper laser.
Alternatively the laser is a solid state laser such as, but not limited to, a Nd:YAG laser, or a Nd:YLF laser, or a Nd:YVO4 laser, or a frequency doubled Nd:YAG or Nd:YLF or Nd:YVO4 laser. The solid state laser may be lamp or laser diode driven. The solid state laser may operate at any harmonic of its fundamental wavelength, for example frequency doubled, tripled, quadrupled or fifth harmonic.
The apparatus may further comprise a separator between the fluid and the hole to control the interaction of the fluid and scattered laser-light with the hole.
The separator may be a solid barrier.
The solid barrier may be perforated with a hole or holes or a slot or slots.
Preferably a hole or a slot is in alignment with the laser beam.
Preferably the solid barrier also prevents the fluid from splashing back or ingressing onto or into the laser drilled hole.
A volume of gas may be used to control the interaction between the hole and the fluid.
Preferably a stream of gas is used as the separator to control the interaction between the hole and the fluid.
The apparatus may further comprise manual or automatic means of loading the workpiece onto the apparatus and unloading the workpiece when the drilling is complete.
The apparatus may also comprise manual or automatic means to position the workpiece with respect to the laser beam so as to drill a hole or holes in the correct position(s).
Preferably a means is provided to position the workpiece on the apparatus to perform the laser drilling. This means may be in the form of a robot, or some form of automatic part-handling system, or a carousel system or a manual loading system.
When the insert makes a fluid seal with the inner surface of the nozzles and when the nozzle must rotate with respect to the insert when changing to the next hole position, then preferably there is a means to lubricate this rotation or there is a manual or automatic means to release this seal, rotate the nozzle with respect to the insert and then remake the seal.
Preferably a means is provided to position the workpiece and apparatus with respect to the laser beam. This may be in the form of a computer or programmable numerical control system with motorised positioning mechanical components.
This system may be used to position the workpiece or position the laser beam or both. It may be a system that manipulates and positions the laser beam by using mirrors or prisms or lenses or gratings or acoustic-optic deflectors or electo-optic devices or fibre optic cables or other optical components.
Preferably a computer or programmable logic controller or some other device is used to control the positioning, loading and laser systems.
Other preferred features of the invention will be apparent from the following description and from the subsidiary claims of the specification.
The invention will now be further described, merely by way of example, in two preferred examples of a method and apparatus for drilling diesel fuel injector nozzles, and by reference to the accompanying drawings, in which: